Mitochondrial Neurogastrointestinal Encephalomyopathy
Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE, Mendelian Inheritance in Man #603041, Genome Database accession #9835128) is a fatal inherited metabolic disorder caused by mutations in a nuclear gene controlling the replication and expression of the mitochondrial genome (Nishino et al., 1999; Hirano et al., 2004). In the past the disorder has also been referred to as:                polyneuropathy, ophthalmoplegia, leukoencephalopathy, and intestinal pseudo-obstruction (POLIP);        myoneurogastrointestinal encephalopathy (MNGIE);        oculogastrointestinal muscular dystrophy (OGIMD);        mitochondrial neurogastrointestinal encephalopathy syndrome;        mitochondrial encepalomyopathy with sensorimotor polyneuropathy, ophthalmoplegia, and pseudo-obstruction (MEPOP);        oculogastrointestinal muscular dystrophy (OGIMD);        thymidine phosphorylase deficiency; and        mitochondrial neurogastrointestinal encephalopathy syndrome.        
In 1994 the name mitochondrial neurogastrointestinal encephalomyopathy was proposed, with the aim of preserving the acronym MNGIE, but to emphasize the mitochondrial abnormalities which are central to the pathogenic mechanism of this disorder (Hirano et al., 1994).
MNGIE is an autosomal recessive disorder of nucleotide metabolism caused by mutations in the nuclear TYMP gene (previously known as ECGF1). This gene encodes thymidine phosphorylase (EC 2.4.2.4), the enzyme required for the normal metabolism of the pyrimidine deoxynucleosides thymidine and deoxyuridine (Hirano et al., 2004). Mutations in the TYMP gene result in a complete or partial absence of thymidine phosphorylase activity, leading to a dramatic accumulation of thymidine and deoxyuridine in tissues and body fluids (Nishino et al., 1999; 2000; Hirano et al., 1994; Spinazzola et al., 2002 Marti et al., 2003; Valentino et al., 2007). Elevated systemic concentrations of these deoxynucleosides is mirrored by elevated intracellular concentrations of their corresponding triphosphates. This perturbs the physiological equilibrium of the four deoxynucleoside triphosphates within the mitochondria, thereby interfering with the normal replication of mtDNA, leading to multiple deletions, somatic point mutations and depletion of mtDNA (Hirano et al., 1994; Marti et al., 2003; Nishigaki et al., 2003; 2004) and ultimately mitochondrial failure (Hirano et al., 1994; Spinazzola et al., 2002; Marti et al., 2003). mtDNA codes for polypeptides, transfer RNA (tRNA) and ribosomal RNA (rRNA) required for the synthesis of enzymes involved in oxidative phosphorylation. The consequent failure of cellular energy production is believed to directly cause the central clinical manifestation, the degeneration of the peripheral nervous system.
Patients with MNGIE usually present during the second decade of life, though patients have presented as early as five months and as late as the fifth decade; the average age is 18.5 years (Nishino et al., 2001). The relatively late-onset is thought to be due to the progressive accumulation of mtDNA defects, with disease becoming apparent once the number of affected mitochondria reaches a critical threshold level. The disease has a homogeneous clinical presentation with gastrointestinal symptoms including early satiety, nausea, chronic abdominal pain, diarrhoea and weight loss. These symptoms are secondary to alimentary dysmotility caused by degeneration of the alimentary peripheral nervous system. Patients generally have a thin body habitus with reduced muscle mass, and cachexia may develop. Episodes of frank intestinal pseudo-obstruction may occur. Some patients develop a hepatopathy with liver steatosis and cirrhosis. Progressive external ophthalmoplegia and peripheral sensorimotor polyneuropathy are invariable. The latter affects the lower limbs initially. On magnetic resonance imaging (MRI) there is, in most cases, diffuse increased T2 signal in the deep white matter of the cerebral hemispheres, but this is usually asymptomatic (Hirano et al., 1994).
Skeletal muscle biopsy may show ragged-red fibres (due to abnormal proliferation of mitochondria in response to defective oxidative phosphorylation), ultrastructurally abnormal mitochondria, and abnormalities of both mitochondrial DNA (mtDNA) and mitochondrial electron transport chain enzymes activities on enzyme analysis (Papadimitriou et al., 1998).
Biochemical studies may show lactic acidosis, indicative of an oxidative phosphorylation defect. Plasma thymidine and deoxyuridine levels are increased to >3 μmo/l and 5 μmol/l, respectively, compared to undetectable levels in healthy unaffected controls (Marti et al., 2003; 2004). Urine concentrations of thymidine and deoxyuridine are also increased (Spinazzola et al., 2002). Thymidine phosphorylase activity in leukcocytes of patients with MNGIE is severely reduced, showing little (<10% of healthy unaffected controls) or no activity (Spinazzola et al., 2002; Marti et al., 2004).
The molecular basis of MNGIE has been determined by PCR amplification and DNA sequence analysis; 52 different mutations in the TYMP gene have been identified since its first description and no predominant one has been reported [The Human Gene Mutation Database (HGMD) at the Institute of Medical Genetics in Cardiff]. A majority of these mutations are missense or nonsense, the others being small deletions, small insertions, and splice-site mutations. Patients are either homozygous or compound heterozygous for the TYMP mutation. Heterozygous carriers of TYMP mutations have 26 to 35% of residual thymidine phosphorylase activity, are asymptomatic and have undetectable levels of plasma thymidine and deoxyuridine (Nishino et al., 1999; Spinazzola et al., 2002).
MNGIE is a relentlessly progressive, degenerative disease with a poor prognosis, and causes a great deal of suffering to affected individuals. Gastrointestinal dysmotility caused by degeneration of the alimentary peripheral nervous system occurs in nearly all patients. The resulting digestive problems include early satiety, problems with swallowing (dysphagia), nausea and vomiting after eating, episodic abdominal distention and pain, and diarrhea. These gastrointestinal problems lead to severe weight loss and a reduced muscle mass. Disability results from the peripheral neuropathy; patients experience weakness of the lower extremities, particularly in hands and feet, numbness and tingling sensations. Other symptoms include ptosis (droopy eyelids), ophthalmoplegia (weakness of muscles which control eye movement), and hearing loss.
At present, there is no recognized specific treatment to prevent or reverse the inexorable clinical deterioration, and clinical management of the symptoms is non-specific and supportive. Survival is generally related to the degree of gastrointestinal involvement, with patients often dying as a result of cachexia, peritonitis, esophageal bleeding, intestinal rupture, or aspiration pneumonia.
In order to assess the mortality of MNGIE, the English language literature on the condition published between October 2005 to October 2010 was searched [Pubmed using search terms: MNGIE; Mitochondrial neurogastrointestinal encephalomyopathy; Chronic intestinal pseudo-obstruction]. The age of death could be determined in nineteen cases, for which the mean was 30.2 years, range 18 to 39 years (Aksoy et al., 2005; Said et al., 2005; Hirano et al., 2006; la Marca et al., 2006; Moran et al., 2006; Valentino et al., 2007; Giordano et al., 2008; Baris et al., 2010; Cardaioli et al., 2010). This compares with Nishino et al (2000) who estimated the mean age of death as 37.6 years; with a range of 26 to 58 years.
Treatments for MNGIE
MNGIE is a rare inherited metabolic disease for which at present there are no EMEA- or FDA-approved therapies. There is a critical requirement to develop a treatment that may prove beneficial.
There are no proven specific treatments for MNGIE. Current treatment is, therefore, symptomatic. Abdominal pain and nausea/vomiting secondary to gastrointestinal dysmotility are almost invariable. These symptoms are treated symptomatically with analgesics, bowel motility-stimulant drugs, anti-emetics and antibiotics for intestinal bacterial overgrowth. In intractable pain, splanchnic nerve or coeliac plexus blockade with bupivicaine has been reported to be helpful (Celebi et al, 2006; Shoffner, 2010). Pain may also occur in the limbs due to peripheral polyneuropathy. Such neuropathic pain is generally treated with centrally acting agents such as amitriptyline or pregabalin. In individuals with MNGIE there is an increased incidence of perforation of the gut. This generally requires emergency abdominal surgery. Malnutrition is a major problem in most individuals with MNGIE. Various forms of parenteral nutrition, including total parenteral nutrition, are frequently required.
Portal hypertension may occur and be complicated by ascites and oesophageal varices. These conditions are treated in the same way as when they occur in other conditions. Drugs that interfere with mitochondrial function should be avoided in individuals with MNGIE. Hepatically metabolized drugs should be administered with care or may be contraindicated depending on liver function. Physiotherapy and occupational therapy input is usually required, particularly to address the neurological aspects of the condition. In children special schooling arrangements may be necessary (Shoffner, 2010). MNGIE is a hereditary condition and individuals with the condition should be offered genetic counseling.
Thymidine and deoxyuridine are freely diffusible across cell membranes and exist in a state of equilibrium between the cellular and plasma compartments, and thus therapeutic strategies which aim to reduce or eliminate the pathological concentrations of plasma thymidine and deoxyuridine may be beneficial to patients with MNGIE. Haemodialysis and continuous ambulatory peritoneal dialysis have been used in an attempt to remove the toxic metabolites (Spinazzola et al., 2002; la Marca et al., 2006; Yavuz et al., 2007); haemodialysis was able to lower plasma nucleoside levels, but there was a rapid re-accumulation to pre-dialysis levels between dialysis sessions (la Marca et al., 2006).
Although peritoneal dialysis was unable to demonstrate a decrease in plasma metabolites, an improvement of clinical symptoms was noted. Infusions of platelets, which contain thymidine phosphorylase, have been shown to reduce circulating levels of thymidine and deoxyuridine in two patients (Lara et al., 2006). However, long-term platelet therapy is not a feasible option due to the short lifespan of transfused platelets, and risks of developing immune reactions and transmission of viral infections.
More recently allogenic stem cell transplantation has demonstrated a partial restoration of white cell thymidine phosphorylase activity, and a reduction or disappearance of plasma thymidine concentrations in patients who successfully engrafted (Hirano et al., 2006). Evidence that clinical benefit can be achieved by correction of the biochemical abnormalities has been shown in a single patient; thirty months post engraftment, it was noted that previously absent tendon reflexes had returned, and an improvement in nerve conduction. Total parenteral nutrition was replaced by a normal diet of 3,000 calories daily and bowel movements had normalised (Hirano et al., 2008).
Allogenic haematopoietic stem cell transplantation (HSCT) offers the possibility of a permanent correction of thymidine phosphorylase deficiency. To date, nine patients world-wide have received 12 allogenic HSCT; a second HSCT being performed in three patients (Halter et al., 2010). The graft sources employed were peripheral blood stem cells, bone marrow and cord blood. Four patients died, two due to transplant related mortality, and two from their disease. The remaining five patients were alive 8-48 months post-transplant and all demonstrated reduction or disappearance of plasma thymidine and deoxyuridine. An improvement of gastrointestinal symptoms and slight improvement of neurological symptoms has been observed in two patients.
Allogenic HSCT is still highly experimental and carries a 44% mortality risk. HSCT can potentially cure, but is limited by the availability of a matched donor. Patients are in a poor clinical condition with a restricted capacity to tolerate transplant-related problems. The administration of HSCT to these patients presents pharmacological challenges in terms of administering drugs with possible mitochondrial toxicity, and the requirement for parental administration due to disturbed gastrointestinal function and impairment of absorption. A published consensus proposal for standardising an approach to allogenic HSCT in MNGIE recommends restricting the recruitment of patients with an optimal donor to those without irreversible end-stage disease (Halter et al., 2010). Thus, for many patients there is no treatment option and clinical management is based on symptom relief and palliation.
The last two decades has seen the introduction of enzyme replacement therapies for the successful treatment of inherited metabolic diseases, including adenosine deaminase deficiency, Gaucher disease and other lysosomal storage disorders. The administration of the missing enzyme, usually by injection, enables the elimination of the pathological substrates which accumulate in these metabolic disorders, translating into clinical benefit. Chemical modifications of the native enzyme are often employed in the manufacturing process to increase protein stability, decrease immunogenicity, and to enable targeting of enzyme to the appropriate cellular compartment. The development of recombinant DNA techniques and over-expressing cells has made it possible to produce quantities of pure enzyme on a commercial scale.
The encapsulation of therapeutic enzymes within autologous erythrocytes is an alternative therapeutic approach for enzyme replacement therapy, and is applicable to disorders where the pathological plasma metabolite is able to permeate the erythrocyte membrane (FIG. 1). Erythrocyte encapsulated enzyme replacement therapy has the advantage of prolonging the circulatory half-life of the enzyme and maintaining therapeutic blood levels, reducing the dosage and frequency of therapeutic interventions, and negating the need for expensive chemical modification.
In July 2006, a single dose of erythrocyte encapsulated thymidine phosphorylase was administered to a seriously ill patient with MNGIE for whom there was no other treatment. The patient was administered 1020 units thymidine phosphorylase encapsulated within 20.25×1010 erythrocytes. At 3 days post infusion, the urinary excretion of thymidine and deoxyuridine decreased to 6% and 13%, respectively of the amounts excreted pre-therapy. In parallel, the plasma concentrations of these metabolites decreased during the first 3 days after infusion. Sadly, the patient died from pneumonia 21 days later. The results of this single compassionate use of this therapeutic approach are published (Moran et al., 2008).